Abstract: A self-priming passive dosing dispenser for issuing, beginning with the first flush, a quantity of a toilet tank additive solution into a toilet tank as the water is draining therefrom while the toilet is flushing. A preferred dispenser comprises a reservoir for containing a quantity of a toilet tank additive type product and in which reservoir a solid type product can be dissolved to form a product solution. Structure is provided for flooding the reservoir through the discharge standpipe upon initial immersion of the dispenser into the toilet tank water to ensure first flush dispensing of product solution. In steady state operation, while the water in the toilet tank is receding from about the dispenser, a predetermined dose-volume of toilet tank water is vacuum-transferred from a measuring cavity located within said dispenser into said reservoir through an inlet conduit, and a substantially equal dose-volume of the product solution is dispensed through a discharge standpipe.